The present invention relates to a turbine fuel pump suitable for use, for example, in fuel supply to an injection valve for an automotive engine.
Typically, the vehicle such as a passenger car is provided with an electronically controlled fuel injection system for supplying fuel to an engine, which comprises an injection valve for injecting fuel to an engine combustion chamber, a fuel pump for delivering to the injection valve fuel within a fuel tank arranged, e.g. in the rear of the vehicle, etc. Recently, because of social requirements of the global environmental protection, there is an increasing demand for an improvement in fuel consumption of the vehicle. Thus, it is an important challenge for the fuel pump driven by an electric motor to achieve an enhancement in efficiency (i.e. reduction in electric power consumption) and a reduction in size and weight.
The fuel pump in general use includes a turbine fuel pump comprising a cylindrical casing for accommodating an electric motor, an upper cover arranged at one end of the casing, a housing arranged at another end of the casing so as to support the motor and having an annular fuel passage between fuel inlet and outlet ports, and an impeller rotatably arranged in the housing and for feeding fuel sucked through the inlet port to the outlet port via the fuel passage while being rotated by the motor.
The impeller is formed like a disc, and has blades arranged circumferentially at the outer periphery and extending radially and blade grooves formed between the blades. Fuel sucked through the inlet port is introduced into the blade grooves via the fuel passage to receive kinetic energy from the blades, and it is then discharged to the passage. Fuel discharged to the fuel passage is circulated through the passage, then introduced again into the blade grooves. Fuel within the passage is increased in pressure by repetition of the inflow and outflow, and discharged through the outlet port.
It is important for enhancement of both of the efficiency of the electric motor and that of the pump portion to improve the efficiency of the fuel pump. Specifically, the impeller is driven by the electric motor which rotates in fuel, producing a torque loss due to viscosity of fuel. When rotating in the housing, the impeller also produces a torque loss due to viscosity of fuel. Those torque losses are increased in proportion to the square of rpm, and thus become very great values when the fuel pump is operated at high rpm, resulting in a reduction in pump efficiency.
Then, a torque loss can be restrained by setting the specifications of the pump portion to allow achievement of a required flow rate at lower rpm. In this case, however, torque required for driving of the impeller is increased.
Moreover, because of requirements of downsizing of the fuel pump, the electric motor has been reduced in size. As described above, generation of high torque at low rpm needs operation of the electric motor in the low-efficiency range. Thus, it is important for enhancement of the pump efficiency to provide not only the specifications of the pump portion to minimize a torque loss, but also the specifications of the electric motor to allow its service in the high-efficiency range.
In connection with the art to improve the efficiency of the turbine fuel pump, various improvements in the impeller have been proposed. One of the improvements is disclosed in JP-A 8-100780 wherein each blade of the impeller has a root portion curved backward as viewed in the direction of rotation of the impeller, and a head portion extending radially outward from a curved portion to incline backward linearly. This shape of the blade allows smooth fuel flow from a blade groove to a passage even in the range of relatively low rpm, preventing a reduction in flow rate with respect to rpm, resulting in enhancement in the low-voltage characteristics and flow-rate controllability.
With the turbine fuel pump disclosed in JP-A 8-100780, as described above, each blade of the impeller has a root portion curved backward as viewed in the direction of rotation of the impeller, and a head portion extending radially outward from a curved portion to incline backward linearly. With this, the impeller allows prevention of the flow rate with respect to rpm in the range of relatively low rpm. However, since the impeller has a head portion inclining backward linearly, outflow of fuel from the blade groove is carried out in the rear direction, providing no higher kinetic energy to fuel. Thus, achievement of relatively great flow rate requires a considerable increase in rpm. This leads to an increase in torque loss in the range of relatively great flow rate, raising a problem of a reduction in pump efficiency.